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United States Patent |
5,349,168
|
Wilen
|
September 20, 1994
|
Microwaveable packaging composition
Abstract
Microwaveable packing compositions exhibiting improved temperature control
are provided. These compositions comprise a dielectric substrate having at
least a portion of at least one of its surfaces coated with a matrix
composition containing susceptor particles as well as particles of a
blocking agent selected from the group consisting of calcium salts, zinc
salts, zinc oxide, lithopone, silica and titanium dioxide. Also disclosed
is a microwaveable ink composition useful for the preparation of such
packaging compositions as well as a process for manufacturing such
packaging compositions.
Inventors:
|
Wilen; Allan S. (Kernersville, NC)
|
Assignee:
|
Zeneca Inc. (Wilmington, DE)
|
Appl. No.:
|
101807 |
Filed:
|
August 3, 1993 |
Current U.S. Class: |
219/730; 99/DIG.14; 219/731; 426/107; 426/113; 426/234; 426/243; 427/407.1; 428/323 |
Intern'l Class: |
H05B 006/80 |
Field of Search: |
219/730,731,10.55 E,10.55 F
99/DIG. 14
426/107,113,234,243
427/407.1
428/323,328,461
206/484
|
References Cited
U.S. Patent Documents
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|
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|
4190757 | Feb., 1980 | Turpin et al. | 219/10.
|
4264668 | Apr., 1981 | Balla | 428/195.
|
4267420 | May., 1981 | Brastad | 219/10.
|
4434197 | Feb., 1984 | Petriello et al. | 427/407.
|
4518651 | May., 1985 | Wolfe | 428/308.
|
4735513 | Apr., 1988 | Watkins et al. | 383/116.
|
4765929 | Aug., 1988 | Shaffer | 252/511.
|
4808780 | Feb., 1989 | Seaborne | 219/10.
|
4825024 | Apr., 1989 | Seaborne | 219/10.
|
4864089 | Sep., 1989 | Tighe et al. | 219/10.
|
4866232 | Sep., 1989 | Stone | 219/10.
|
4876423 | Oct., 1989 | Tighe et al. | 219/10.
|
4878765 | Nov., 1989 | Watkins et al. | 383/116.
|
4892782 | Jan., 1990 | Fisher et al. | 428/240.
|
4904836 | Feb., 1990 | Turpin et al. | 219/10.
|
4914266 | Apr., 1990 | Parks et al. | 219/10.
|
4927991 | May., 1990 | Wendt et al. | 219/10.
|
4933526 | Jun., 1990 | Fisher et al. | 219/10.
|
4935252 | Jun., 1990 | Huang et al. | 426/107.
|
4937173 | Jun., 1990 | Kanda et al. | 430/281.
|
4943456 | Jul., 1990 | Pollart et al. | 428/34.
|
4959516 | Sep., 1990 | Tighe et al. | 219/10.
|
4962000 | Oct., 1990 | Emslander et al. | 219/10.
|
4970358 | Nov., 1990 | Brandberg et al. | 219/10.
|
4970360 | Nov., 1990 | Pesheck et al. | 219/10.
|
4972058 | Nov., 1990 | Benson et al. | 219/10.
|
5002826 | Mar., 1991 | Pollart | 428/323.
|
5006405 | Apr., 1991 | Watkins et al. | 428/323.
|
5021293 | Jun., 1991 | Huang et al. | 428/328.
|
5038009 | Aug., 1991 | Babbitt | 219/10.
|
5041295 | Aug., 1991 | Perry et al. | 426/107.
|
5059279 | Oct., 1991 | Wilson | 156/651.
|
5079083 | Jan., 1992 | Watkins et al. | 428/323.
|
5079397 | Jan., 1992 | Keefer | 219/10.
|
5095186 | Mar., 1992 | Russell et al. | 219/10.
|
5118747 | Jun., 1992 | Pollart et al. | 524/424.
|
5132144 | Jul., 1992 | Parks | 427/210.
|
5149396 | Sep., 1992 | Wilson | 156/656.
|
5164562 | Nov., 1992 | Huffman et al. | 219/10.
|
5171594 | Dec., 1992 | Babbitt | 426/107.
|
5175031 | Dec., 1992 | Ochocki | 428/34.
|
5177332 | Jan., 1993 | Fong | 219/10.
|
5180894 | Jan., 1993 | Quick et al. | 219/10.
|
5183787 | Feb., 1993 | Seaborne | 501/143.
|
5185506 | Feb., 1993 | Walters | 219/10.
|
5195829 | Mar., 1993 | Watkins et al. | 383/100.
|
Foreign Patent Documents |
242952 | Oct., 1987 | EP.
| |
0357008 | Aug., 1989 | EP | .
|
Primary Examiner: Reynolds; Bruce A.
Assistant Examiner: Hoang; Tu
Parent Case Text
This is a continuation of copending application Ser. No. 07/545,330 filed
on Jun. 27, 1990, now abandoned.
Claims
What is claimed is:
1. A microwaveable package comprising:
(A) a dielectric substrate substantially transparent to microwave
radiation; and
(B) a coating on at least a portion of at least one surface of the
substrate, said coating comprising a matrix comprised of a dielectric
material having dispersed therein:
(i) a sufficient amount of particles of a microwave susceptor material
whereby heat is generated when the coating is exposed to microwave
radiation; and
(ii) a sufficient amount of particles of a blocking agent selected from the
group consisting of calcium salts, lithopone, silica and titanium dioxide
whereby when said coating composition is exposed to a preselected dosage
of microwave radiation, the heat generated by the susceptor material is
controlled within a preselected range.
2. A microwaveable package in accordance with claim 1 wherein said matrix
is composed of a material selected from the group consisting of
polyacrylates, polymethacrylates, polyesters, polyester copolymers,
copolyester polyurethanes, epoxy resins, polycarbonates,
polyethersulfones, polyarylsulfones, polyamide-imides, polyamides,
poly-4'4-isopropylidene diphenylene carbonate, polyetheretherketones,
imidazoles, oxazoles and thiazoles.
3. A microwaveable package in accordance with claim 2 wherein said matrix
material is selected from the group consisting of acrylic polymers and
copolymers.
4. A microwaveable package in accordance with claim 3 wherein said matrix
material is composed of poly(methyl methacrylate) or poly(ethyl
methacrylate).
5. A microwaveable package in accordance with claim 1 wherein the microwave
susceptor material is selected from the group consisting of nickel,
antimony, copper, molybdenum, bronze, iron, chromium, tin, zinc, silver,
gold, aluminum, graphite, silicon carbides and ground metallized films.
6. A microwaveable package in accordance with claim 5 wherein said
microwave susceptor material is an alloy of copper, zinc and nickel, or a
leafing aluminum powder.
7. A microwaveable package in accordance with claim 1 wherein the blocking
agent is selected from the group consisting of calcium carbonate, calcium
sulfate, silica and titanium dioxide.
8. A microwaveable package in accordance with claim 7 wherein said blocking
agent is selected from the group consisting of titanium dioxide and
calcium carbonate.
9. A microwaveable package in accordance with claim 1 wherein:
A) said matrix material is selected from the group consisting of
poly(methyl methacrylate) and poly(ethyl methacrylate);
B) said microwave susceptor material is selected from the group consisting
of alloys of copper, zinc and nickel and leafing aluminum powder; and
C) said blocking agent is selected from the group consisting of titanium
dioxide and calcium carbonate.
10. A microwaveable packaging ink composition comprising a liquid carrier
having incorporated therein:
(A) a dielectric polymeric material substantially transparent to microwave
radiation;
(B) particles of microwave susceptor material; and
(C) particles of a blocking agent selected from the group consisting of
calcium salts, lithopone, silica and titanium dioxide; the microwave
susceptor material and blocking agent being present in amounts whereby
when a coating formed by application of said ink composition is subjected
to a preselected dosage of microwave radiation, the heat generated by the
coating is controlled within a preselected range.
11. A microwaveable packaging ink composition in accordance with claim 10
wherein the dielectric polymeric material is selected from the group
consisting of polyacrylate, polymethyacrylates, polyesters, polyester
copolymers, copolyester polyurethanes, epoxy resins, polycarbonates,
polyethersulfone, polyarylsulfanes, polyamide-imides, polyimides,
poly-4'4-isopropylidene diphenylene carbonate polyetheretherketones,
imidazoles, oxazoles and thiazoles.
12. A microwaveable packaging ink composition in accordance with claim 11
wherein said dielectric polymeric material is selected from the group
consisting of acrylic polymers and copolymers.
13. A microwaveable packaging ink composition in accordance with claim 12
wherein said dielectric polymeric material is composed of poly(methyl
methacrylate) or poly(ethyl methacrylate).
14. A microwaveable packaging ink composition in accordance with claim 10
wherein the microwave susceptor material is selected from the group
consisting of nickel, antimony, copper, molybdenum, bronze, iron,
chromium, tin, zinc, silver, gold, aluminum, graphite, silicon carbides
and ground metallized films.
15. A microwaveable packaging ink composition in accordance with claim 14
wherein said microwave susceptor material is an alloy of copper, zinc and
nickel, or a leafing aluminum powder.
16. A microwaveable packaging ink composition in accordance with claim 10
wherein the blocking agent is selected from the group consisting of
calcium carbonate, calcium sulfate, silica and titanium dioxide.
17. A microwaveable packaging ink composition in accordance with claim 16
wherein said blocking material is selected from the group consisting of
titanium dioxide and calcium carbonate.
18. A microwaveable packaging ink composition in accordance with claim 10
wherein:
(A) said dielectric polymeric material is selected from the group
consisting of poly(methyl methacrylate) and poly(ethyl methacrylate);
(B) said microwave susceptor material is selected from the group consisting
of alloys of copper, zinc and nickel and leafing aluminum powder; and
(c) said blocking agent is selected from the group consisting of titanium
dioxide and calcium carbonate.
19. A microwaveable packaging ink composition in accordance with claim 10
wherein said liquid carrier comprises water.
20. A microwaveable packaging ink composition in accordance with claim 10
wherein said liquid carrier comprises an organic solvent.
21. A process of manufacturing a microwaveable packaging composition
comprising the steps of:
(A) preparing a coating composition comprising a dielectric material having
dispersed therein
(i) particles of a microwave susceptor material; and
(ii) particles of a blocking agent selected from the group consisting of
calcium salts, lithopone, silica and titanium dioxide, and said microwave
susceptor material and blocking agent being present in an amount whereby
when a coating formed by application of said coating composition is
subjected to a preselected dosage of microwave radiation, the heat
generated by the susceptor material is controlled within a preselected
range; and
(B) coating said coating composition onto at least a portion of at least
one surface of a dielectric substrate substantially transparent to
microwave radiation.
22. A process in accordance with claim 21 wherein the coating composition
is applied by gravure printing.
23. A microwaveable package comprising:
(A) a dielectric substrate substantially transparent to microwave
radiation; and
(B) a coating on at least a portion of at least one surface of said
substrate, said coating comprising a matrix comprised of a dielectric
material having dispersed therein:
(i) particles of a microwave susceptor material; and
(ii) particles of a blocking agent selected from the group consisting of
calcium salts, lithopone, silica and titanium dioxide,
wherein the microwave susceptor material comprises between about 3 to about
80 percent by weight of the total weight of the microwave susceptor
material, the dielectric material and the blocking agent; and
wherein the weight ratio of said blocking agent to microwave susceptor
material is 1:4 or more.
24. A microwaveable packaging ink composition comprising a liquid carrier
having incorporated therein:
(A) a dielectric polymeric material substantially transparent to microwave
radiation;
(B) particles of microwave susceptor material; and
(C) particles of a blocking agent selected from the group consisting of
calcium salts, lithopone, silica and titanium dioxide;
wherein the microwave susceptor material comprises between about 3 to about
80 percent by weight of the total weight of the dielectric polymeric
material, the microwave susceptor material and the blocking agent; and
wherein the weight ratio of said blocking agent to microwave susceptor
material is 1:4 or more.
25. A process of manufacturing a microwaveable packaging composition
comprising the steps of:
(A) preparing a coating composition comprising a dielectric material having
dispersed therein
(i) particles of a microwave susceptor material; and
(ii) particles of a blocking agent selected from the group consisting of
calcium salts, lithopone, silica and titanium dioxide,
wherein the microwave susceptor material comprises between about 3 to about
80 percent by weight of the total weight of the dielectric material, the
microwave susceptor material and the blocking agent; and wherein the
weight ratio of said blocking agent to microwave susceptor material is 1:4
or more; and
(B) coating said coating composition onto at least a portion of at least
one surface of a dielectric substrate substantially transparent to
microwave radiation.
Description
FIELD OF THE INVENTION
In one aspect, this invention is directed to a microwaveable package
comprising a dielectric substrate substantially transparent to microwave
radiation having at least one portion of at least one surface thereof
coated with a coating composition comprising a dielectric matrix having
dispersed therein (A) a sufficient amount of particles of microwave
susceptor material such that heat will be generated when such coating
composition is exposed to microwave radiation; and (B) a sufficient amount
of particles of a blocking agent selected from the group consisting of
calcium salts, zinc salts, zinc oxide, lithopone, silica and titanium
dioxide, such that when such coating composition is subjected to a
preselected dosage of microwave radiation the heat generated by the
susceptor material is controlled with a preselected range.
In another aspect, this invention is directed to a microwaveable packaging
ink composition comprising a liquid carrier having incorporated therein
(A) a dielectric polymeric material substantially transparent to microwave
radiation; (B) particles of microwave susceptor material; and (C)
particles of at least one blocking agent selected from the group
consisting of calcium salts, zinc salts, zinc oxide, lithopone, silica and
titanium dioxide; such microwave susceptor material and blocking agent
being present in amounts such that when the coating formed by the
application of such ink is subjected to a preselected dosage of microwave
radiation, the heat generated by the susceptor material is controlled
within a preselected range.
In yet another aspect, this invention is directed to a process for
manufacturing such a microwaveable packaging composition.
BACKGROUND OF THE INVENTION
The recent proliferation of microwave ovens for the preparation and cooking
of food has created a need for the production of improved packaging in
order to render certain types of food more amenable to microwave cooking.
Thus, for example, certain foods, such as popcorn, may not absorb enough
microwave energy to generate sufficient heat to pop or cook. Other foods
require browning or crisping of their surfaces, results which cannot
ordinarily be achieved by the use of conventional food packaging
compositions in microwave ovens.
In order to meet this need for improved microwaveable packing, several
different approaches have been proposed.
One general approach has been to form a multi-layered wrap-type composition
composed of an energy absorbing susceptor material and a plastic film or
other dielectric substrate. Thus, for example, U.S. Pat. No. 4,267,420
(Brastad) discloses a packaging material which is a plastic film or other
dielectric substrate having a thin semiconducting coating, preferably of
evaporated aluminum. Somewhat similarly, U.S. Pat. No. 4,434,197
(Petriello et al) shows a multi-layered laminated microwaveable packaging
material including outside layers of polytetrafluoroethylene, two
intermediate layers of pigmented polytetrafluoroethylene and a central
layer of polytetrafluoroethylene having dispersed therein particles of an
energy absorbing susceptor material such as graphite, ferric oxide or
carbon.
A second general approach which has been proposed involves the dispersion
of particles of a microwave absorbing composition in a polymeric or
ceramic-type material matrix. Thus, for example, U.S. Pat. No. 4,190,757
(Turpin et al) discloses a microwaveable package composed of a non-lossy
dielectric sheet material defining a container body and a lossy microwave
absorbitive heating body connected thereto, such heating body typically
comprising particles of microwave absorbitive susceptor material
(including zinc oxide, germanium oxide, iron oxide, alloys of metals such
as of manganese, aluminum and copper, oxides, carbon and graphite) in a
ceramic-type binder (such as cement, plaster of paris or sodium silicate).
Somewhat similarly, U.S. Pat. No. 4,518,651 (Wolfe) shows microwaveable
composite materials comprising a polymeric matrix having electronically
conductive particles dispersed therein, which matrix is bound to a porous
substrate. This patent teaches that it is critical that at least some of
the polymer matrix beneath the surface of the substrate be substantially
free of electronically conductive particles and be intermingled with the
substrate.
European Patent Publication 242,952 discloses a microwaveable packaging
material which is a composite comprising a dielectric material (e.g.,
polyethylene terephthalate film) coated with a mixture of an electrically
conductive metal or metal alloy in flake form in a dielectric matrix. This
patent indicates that to obtain optimum heating performance
reproductibility, circular flakes with flat surfaces and smooth edges
should be employed. Somewhat similarly, U.S. Pat. No. 4,866,232 (Stone)
discloses a food package for use in a microwave oven, such package being
produced by the deposition of a metallized ink consisting of metal
particles suspended in an ink-like substance onto a container formed from
a heat resistant material which is pervious to microwaves.
While many of the above and similar microwaveable packaging compositions
will function to convert microwave energy into heat, there is still a need
for improved packaging materials. Thus, many proposed microwaveable
packaging materials tend to heat uncontrollably in a microwave oven,
leading to charring or even arcing, ignition and burning of the packaging
material. Other materials are not capable of generating sufficient heat
quickly, while several materials, while functioning desirably, are
economically prohibitive for widespread use.
Accordingly, it is an object of this invention to provide a microwaveable
package which provides for increased control of the heat generated by
exposure to microwaves.
It is a further object of this invention to provide a microwaveable
packaging ink composition which when deposited on a dielectric substrate
will offer improved control of the heat generated upon exposure to
microwave radiation.
It is yet a further object of this invention to provide a microwaveable
packaging ink composition which can be economically employed.
It is an additional object of this invention to provide a method of
economically producing a microwaveable package which provides increased
control of the heat generated upon exposure to microwave radiation.
These objects, and other additional objects, will become more fully
apparent from the following description and accompanying Examples.
SUMMARY OF THE INVENTION
In one aspect, this invention is directed to a microwaveable package
comprising:
(A) a dielectric substrate substantially transparent to microwave
radiation; and
(B) a coating on at least a portion of at least one surface of such
substrate, said coating comprising a matrix comprised of a dielectric
polymeric material having dispersed therein:
(i) a sufficient amount of particles of microwave susceptor material such
that heat will be generated when such coating is exposed to microwave
radiation; and
(ii) a sufficient amount of particles of a blocking agent selected from the
group consisting of calcium salts, zinc salts, zinc oxide, lithopone,
silica and titanium dioxide such that when such coating is exposed to a
preselected dosage of microwave radiation the heat generated by the
susceptor material is controlled within a preselected range.
In another aspect, this invention is directed to a microwaveable packaging
ink composition comprising a liquid carrier having incorporated therein:
(A) a dielectric polymeric material substantially transparent to microwave
radiation;
(B) particles of a microwave susceptor material; and
(C) particles of a blocking agent selected from the group consisting of
calcium salts, zinc salts, zinc oxide, lithopone, silica and titanium
dioxide;
such microwave susceptor and blocking agent being present in amounts such
that when the coating formed by the application of such ink is subjected
to a preselected dosage of microwave radiation, the heat generated by the
coating is controlled within a preselected range.
In yet another aspect, this invention is directed to a process of
manufacturing a microwaveable package comprising the steps of:
(A) preparing a coating composition comprising:
(i) a dielectric polymeric material;
(ii) particles of microwave susceptor material; and
(iii) particles of a blocking agents selected from the group consisting of
calcium salts, zinc salts, zinc oxide, lithopone, silica and titanium
dioxide;
such microwave susceptor material and blocking agent material being present
in amounts such that when a coating formed by the application of such
coating composition is subjected to a preselected dosage of microwave
radiation, the heat generated by the susceptor material is controlled
within a preselected range; and
(B) coating such composition onto at least a portion of at least one
surface of a dielectric substrate substantially transparent to microwave
radiation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The microwaveable package of this invention is comprised of a dielectric
substrate substantially transparent to microwave radiation having at least
a portion of at least one surface thereof coated with a coating
composition comprising a dielectric polymeric matrix having incorporated
therein (A) particles of a microwave susceptor material; and (B) particles
of a blocking agent.
In general, the dielectric substrate may be any material having sufficient
thermal and dimensional stability to be useful as a packaging material at
the high temperatures which may be desired for browning or rapidly heating
foods in a microwave oven (e.g., as high as 150.degree. C. and above).
Useful substrates include polymeric films, for example, polyester films
such as polyethylene terephthalate films as well as polymethylpentene
films, and films of other thermally stable polymers such as polyarylates,
polyamides, polycarbonates, polyetherimides, polyimides and the like.
Moreover, porous structures such as paper or non-woven materials can also
be employed as substrates so long as the required thermal and dimensional
stability is satisfied. For flexible packaging, the substrate is
preferably about 8 to 50 micrometers thick. Thicker, non-flexible
materials, such as found in trays, lidding, bowls and the like, may also
be employed.
As previously indicated, the substrate must have sufficient dimensional
stability at the elevated temperatures involved in microwave cooking to
prevent distortion of the substrate which may result in non-uniform
cooking from loss of intimate contact of the packaging material with the
food to be cooked. Substrates normally lacking such high temperature
dimensional stability can be used if they are laminated with yet another
substrate layer meeting the thermal stability requirements of the original
substrate. The lamination can be accomplished either by taking advantage
of the adhesive properties of the thermoplastic matrix coating on the
original substrate or by using any number of conventional adhesives to aid
in forming a stable laminate. For example, a polyester copolymer coated
polyethylene terephthalate film can be thermally sealed to another
polyester film or to paper or heavier ovenable paperboard. Alternatively,
another adhesive can be applied from solution prior to lamination to
increase the strength of the laminate. These supplemental adhesives can be
selected from a number of commercially available candidates with required
thermal stability. These include copolyesters, copolyester-polyurethanes
and cyanoacrylates.
The dielectric polymeric material forming the matrix of the coating
composition formed in the practice of this invention may be composed of a
variety of materials which, when deposited onto a suitable substrate,
exhibit sufficient thermal stability to allow for dimensional integrity of
the final packaging material at the elevated temperatures associated with
microwave cooking of food.
The dielectrical properties at 915 megahertz and 2450 megahertz of the
matrix formed by the deposition of the polymeric material upon the
packaging substrate is an important variable in terms of the heat
generated in unit time at 2450 MHz. Specifically, the dielectric matrix
should, in general, possess a relative dielectric constant of between
about 2.0 and about 10, preferably of between about 2.1 and about 5, and
should generally possess a relative dielectric loss index of between about
0.001 and about 2.5, preferably of between about 0.01 to 0.6. The matrix
also preferably displays adhesive characteristics to the substrate as well
as to any additional substrate to which the composite may be laminated to
increase dimensional stability.
Illustrative of suitable matrix materials are polyacrylates,
polymethacrylates, polyesters, polyester copolymers, curable resins such
as copolyester-polyurethanes and epoxy resins, polycarbonates,
polyethersulfones, polyarylsulfones, polyamide-imides, polyimides,
polyetheretherketones, poly-4'4-isopropylidene diphenylene carbonate,
imidazoles, oxazoles, and thiazoles. These materials may be crystalline or
amorphous.
Preferred matrix materials include acrylic polymers and copolymers such as
polymethacrylates, polyacrylates, styrene-acrylate copolymers, and
styrene-methacrylate copolymers. Particularly preferred polymeric
materials are poly(methyl methacrylate) and poly(ethyl methacrylate)
having a molecular weight of between about 1,500 and about 20,000.
The microwave susceptor materials employed in the practice of this
invention include any materials which are capable of absorbing the
electric or magnetic portion of the microwave field energy and converting
that energy into heat. Suitable materials include metals such as powdered
nickel, antimony, copper, molybdenum, bronze, iron, chromium, tin, zinc,
silver, gold and aluminum. Other conductive materials such as graphite and
semi-conductive materials such as silicon carbides and magnetic material
such as metal oxides (if available in particulate form) may also be
employed as susceptor materials. Ground metallized films may also be
utilized. Particularly preferred susceptor materials include alloys of
copper, zinc and nickel sold under the designation SF-401 by Obron; as
well as leafing aluminum powder.
The susceptor materials employed in the practice of this invention are in
particulate form. Such particles may be flakes or powders. The size of
such particles will vary in accordance with a number of factors, including
the particular susceptor material selected, the amount of heat to be
generated, the manner in which the coating composition is to be applied;
and the like.
Typically, however, when such coating compositions are to be applied in the
form of inks, due to limitations of the printing processes such powders
will have diameters of no more than about 50 microns. In general, in such
circumstances, particle sizes of between about 0.1 and about 25 microns
are preferably employed. When the susceptor materials are employed in the
form of flakes, (e.g., such as in the form of leafing aluminum) such
flakes are typically of those sizes of flakes routinely used in the
gravure ink art for the printing of metallic coatings.
The blocking agent employed in the practice of this invention comprises at
least one member of the group consisting of calcium salts, zinc salts,
zinc oxide, lithopone, silica and titanium dioixde. Preferred blocking
agents include calcium carbonate, calcium sulfate, zinc oxide, silica and
titanium dioxide and calcium carbonate, with calcium carbonate being most
preferred.
The blocking agents employed in the practice of this invention are
typically employed in particulate form. The particle size of such blocking
agents is generally limited by the particular coating process employed,
and when such coating is applied in the form of an ink, such particle size
is typically less than about 50 microns, with particle sizes of between
about 0.1 and about 25 microns being preferred for most blocking agents.
When calcium carbonate is employed as the blocking agent, particle sizes
of between about 1 and about 10 microns are more preferred, with particle
sizes of between about 3 and about 7 microns being most preferred.
While not wishing to be held to any particular theory, applicant has found
that the presence of such blocking agent controls the amount of heat
generated by the susceptor material. By controlling the ratio and amount
of blocking agent and susceptor, and/or by varying the thickness of the
ink applied, the amount of heat generated by a preselected dosage of
microwave radiation may be consistently controlled within a preselected
range.
Variables which must be taken into account for determining the precise
ratios of susceptor to blocking agent needed for any particular use
include the physical size, shape and surface characteristics of the
susceptor and blocking agent particles contained in the coating
composition, the amount of coating composition to be applied to the
substrate, and the portion size as well as the food to be cooked in such
application. By so altering these variables as well as the
susceptor:blocking agent ratio employed, one of ordinary skill can easily
regulate the compositions of this invention to heat to high temperatures
in a controlled manner in relatively short periods of time in conventional
microwave ovens, e.g., to temperatures of about 150.degree. C. or above,
preferably 190.degree. C. or above in 120 seconds when subjected to
microwave energy generated in dosages typically produced by such ovens,
e.g. at 550 watts at 2450 megahertz.
The susceptor level in the matrix will generally range from about 3 to
about 80% by weight of the combined susceptor blocking agent/matrix
composition. As noted above the optimum levels of susceptor material and
of blocking agent incorporated into the coating compositions of this
invention will depend upon a number of factors, depending upon the
ultimate end use employed. However, it has been found that, in many
instances, weight ratio of 1:4 or more of blocking agent:susceptor
material will effectively prevent heating of the coating composition when
subjected to dosages of microwave radiation generated by conventional
microwave ovens. Lower ratios of blocking agent to receptor material will
result in higher temperatures.
One of ordinary skill in the art can easily determine optimum ratios for
any particular application using routine experimentation, such as that
described in the Examples hereto, wherein the calories generated by a
particular dosage of microwave radiation are measured for particular
coating compositions of given thickness.
The polymeric material is present in an amount sufficient to form a matrix
for the blocking agent and susceptor material.
In addition to the blocking agent, polymeric material, liquid carrier and
susceptor material the coating composition employed in the microwaveable
package of this invention may optionally contain other conventional
additives such as surface modifiers such as waxes and silicones, antifoam
agents leveling agents, surfactants, colorants such as dyes and pigments
and the like, which additives are well known to those of ordinary skill in
the art.
The microwaveable packaging ink composition of this invention is comprised
of a liquid carrier having dispersed or dissolved therein (A) a
matrix-forming dielectric polymeric material substantially transparent to
microwave radiation; (B) particles of a susceptor material; and (C)
particles of a blocking agent.
The liquid carriers which may be employed include those organic solvents
conventionally employed in the manufacture of ink as well as water and
mixtures of one or more of the foregoing. Illustrative of such solvents
are liquid acetates such as isopropyl acetate and the like; alcohols such
as isopropanol, butanol and the like; ketones such as methyl ethyl ketone
and the like; and aromatic hydrocarbons such as toluene and the like.
Particularly preferred solvents include water, isopropyl acetate and
mixtures of isopropyl acetate with toluene.
When the ink composition of this invention comprises an aqueous carrier,
such composition typically further comprises one or more surfactant and/or
dispersant. Thus, desirable results have been obtained employing a
combination of an ethoxylated nonyphenol such as Tergitol NP-40, available
from Union Carbide; a disperant such as Disperbyk 182 available from Byk
Chemie; a wetting agent and antifoaming agent such as Surfynol 104 A. from
Air Products; and a protective colloid such as Anti-Terra 207 available
from Byk Chemie. In addition, it may often be desirable to add a defoaming
agent such as No Foam available from Shamrock. The amounts of each of such
component which may be readily determined by one of the ordinary skill in
the art employing routine experimentation.
The packaging composition of this invention can be manufactured by a number
of methods. In one method, the dielectric matrix may be dissolved or
dispersed in any number of common organic solvents such as
tetrahydrofuran, methylene chloride, ethyl acetate, methyl ethyl ketone or
similar solvents, and then the susceptor and blocking agent dispersed in
water or in this solution. Such solution is then applied to the substrate
by any number of coating processes such as metered doctor roll coating,
gravure coating, reverse roll coating or slot die coating. The liquid is
driven off after application of the coating by conventional oven drying
techniques to form the final coating composition.
A second technique which may be employed is useful when melt stable
matrices are employed. The matrix material is melted in conventional
equipment and the susceptor particles blended with the melt. This mixture
may then be extrusion or melt coated on the substrate.
The susceptor/blocking agent/matrix may be applied to the substrate in
patterns that would allow a variety of temperature properties in a single
sheet of composite material. These patterns may comprise coating
compositions having varying susceptor to blocking agent ratios or may
comprise coating compositions of various thicknesses or both.
The microwaveable compositions of this invention may be economically
manufactured such that they are commercially acceptable for mass
production. Moreover, such compositions will provide enhanced control of
the temperature produced in the microwave oven.
EXAMPLES
The following Examples are intended to further illustrate the invention and
are not intended to limit the scope of the invention in any manner
whatsoever.
Examples 1 and 2 and Comparative Experiment A
In order to show the degree of control provided by the inclusion of a
blocking agent, two ink compositions were prepared by combining the
following components in a blender:
______________________________________
Comp.
Composition Ex. 1 Ex. 2 Exp. A
______________________________________
Isopropyl Acetate 16.8 25.2 33.6
Toluene 4.2 6.3 8.4
Poly(butyl methacrylate)
9.0 13.5 18.0
(Neocryl #873, ICI Americas Inc.)
Susceptor Material (SF-401,
40.0 40.0 40.0
an alloy of copper, nickel and
zinc, Obran)
Titanium Dioxide (R900, duPont)
30.0 15.0
0-
______________________________________
These compositions were applied to the reverse side of carton stock
employing a gravure press using a #6 Meyer bar. These samples were placed
coated-side down in a Samruns 450 Watt Microwave over at high setting. The
sample produced from the formulation of Comparative Experiment A ignited
in less than 5 seconds whereas those samples produced from the
formulations of Examples 1 and 2 showed darking on their face side after
60 seconds of exposure, indicating that high heat had been generated
without ignition.
Example 3 and Comparative Experiment B
Employing a mixer, two ink compositions were prepared comprising the
following parts by weight of the below-listed materials:
______________________________________
Comparative
Composition Ex. 3 Exp. B
______________________________________
Isopropyl Acetate 36.8 38.8
poly(methyl methacrylate)
15.8 16.6
(SCX-611, S. C. Johnson)
Aluminum leafing powder
42.3 44.6
(XI-1136, Alcon)
Calcium Carbonate 5.1
0-
(Camelwite)
______________________________________
A piece of carton stock was coated with the compositions of Example 3 on
one portion and with the composition of Comparative Experiment A on an
adjacent portion making a double bump bar down using a number ten Meyer
bar. The stock was cut into 3 inch.times.3 inch squares, with one-half
such squares being coated with the blocking agent containing formulation
of Example 3 and the other half being coated with the formulation of
Comparative Experiment B. A slice of thin white bread was placed between
two such samples with the printed side away from the bread and the
coatings aligned such that each half of the slice was sandwiched between
identical formulations. This configuration was covered by paper towels and
placed in a Cober Test Oven (Model LBM1.2A) at 45 seconds with the
turntable on and the stirrer running. It was found that under these
conditions that that section of the bread which was sandwiched between the
coatings of Example 3 had not turned color whereas that section of the
bread which was between the coatings of Comparative Experiment B had
darkened considerably.
Examples 4-7 and Comparative Experiment C
In order to show the efficacy of several other materials as blocking agent,
several additional formulations were prepared as follows using a blender:
______________________________________
Example 4 5 6 7
______________________________________
Isopropyl Acetate 19.61 19.61 19.61 19.61
Isopropyl Alcohol 4.90 4.90 4.90
0-
Toluene
0-
0-
0-
0-
Poly(methyl methacrylate)
10.49 10.49 10.49 10.49
(Neocryl #B735, ICIAm Inc.)
Microwave susceptor
50.00 50.00 50.00 50.00
(SF-40), a copper/zinc/nickel
alloy available from Obron
Lithopone (stoichiometric
15.00
0-
0-
0-
mixture of Zn sulfide and Ba
Sulfate)
Sachtolith (mixture of
0- 15.00
0-
0-
Zn sulfate and Zn oxide)
Calcium Carbonate
0-
0- 15.00
0-
(Camelwite)
Fumed Silica
0-
0-
0- 15.00
(Syloid #G20, WRGrace)
______________________________________
Samples of carton stock were printed on their reverse side using a Number 6
Meyer Bar. The samples were then cut into 3 inch squares.
In order to test the effectiveness of the blocking agents in controlling
the temperatures generated, a sample square was placed with the printed
side down under a 250 ml beaker containing 50 grams of water which had an
initial temperature of 68.degree. F. in a Cober Test Oven. The oven was
run at 600 watts for 30 seconds with the turntable on, the stirrer on and
the vent open. The amount of heat generated was calculated by measuring
the increase in temperature of the water. As a control, 50 grams of water
alone was heated in this manner. The average of five runs of each Example
is summarized below:
______________________________________
Example or
Comparative Calories
Experiment Blocking Agent
Absorbed
______________________________________
C None (water only)
3950
4 ZnS/BaSO.sub.4
3600
5 ZnSO.sub.4 /ZnO
3500
6 CaCO.sub.3 3500
7 Silica 3550
______________________________________
The above results demonstrate that under these conditions, zinc salts, zinc
oxide, lithopone and silica all demonstrate the ability to regulate the
heat generated by the susceptor material.
Example 8
Employing a blender, the following ingredients were blended to from an
aqueous based liquid vehicle:
______________________________________
Ingredient Part by Weight
______________________________________
Water 32.80
Defoaming Agent 0.75
(No Foam; Shamrock)
Nonylphenal Ethoxylate (40 moles)
9.37
(Tergitol NP-40; Union Carbide)
Dispersent 1.88
(Disperbyk 182)
Alkylolalumium Salt of
6.50
Unsaturated Fatty Acid
(Surfynol 104 A; Air Products)
Protective Colloid 3.70
(Anti-Terra 207; Byk Chemie)
Acrylic Resin 22.50
Stryrene/Acrylic Copolymer
22.50
______________________________________
The parts by weight of the above vehicle were blended with 2 parts of
calcium carbonate (camel wite) and 20 parts of Obran-Atlantic aluminim
extra brilliant No. 103 non-leafing powder. The resulting water base ink
was printed onto carton stock. A sample of thin white bread was placed
between two samples, printed side out and placed into a Cober Test Oven.
The oven was run for 45 seconds at 70 percent power with the vent open,
stirrer on and turn table running. This treatment resulted in a controlled
toasting of the slice of bread.
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